170 REACTION OF THE INTERIOR OF THE EARTH 



as consisting of unoxidised masses of the metallic bases 

 of earths and alkalies ; and it is supposed that water 

 and air gain access to these and thus that volcanic 

 activity is excited. Volcanoes do, indeed, send forth 

 great quantities of aqueous vapour into the atmo- 

 sphere ; but the hypothesis of a penetration of water 

 to the volcanic hearth would have to encounter great 

 difficulties in respect to the mutual pressure ( 235 ) 

 of the external column of water and the internal lava ; 

 and the absence during eruptions, or at least the 

 great rarity of ignited hydrogen, which is not ade- 

 quately replaced by the formations of hydrochloric 

 acid, ammonia., and sulphuretted hydrogen, ( 236 ) led 

 the illustrious originator of this hypothesis sponta- 

 neously to relinquish it.( 237 ) 



According to the third view, which is that of the ac- 

 complished South-American traveller, Boussingault, 

 a want of continuity in the masses of trachyte and 

 dolerite, of which the upheaved volcanoes of the chain 

 of the Andes are composed, is regarded as^ a principal 

 cause of many and very widely acting earthquakes in 

 those regions. The colossal cones and dome-shaped 

 summits of the Cordilleras are in this view supposed 

 to have been by no means upheaved in a state of soft- 

 ness or semi-fluidity, but are regarded as consisting of 

 enormous angular fragments, which have been pushed 

 up and heaped one upon another. In the course of 

 these operations it would necessarily have followed 

 that, in the vast piles so formed, great intervals and 

 cavities would have occurred, and that by subsidence, 

 and by the occasional fall of imperfectly supported 

 masses, commotions would ensue. ( 238 ) 



